Waveguide multilayer holographic data storage

ABSTRACT

The invention provides a method and apparatus for providing a high information capacity, high data rate and short access time simultaneously. The method and apparatus include a multilayer waveguide holographic carrier, a multilayer waveguide holographic data storage system, a multilayer waveguide hologram reading method with random data access, and a process and apparatus for recording matrix waveguide hologram layers and assembling a multilayer carrier.

FIELD OF THE INVENTION

The present invention relates to volume holographic data storage andmore particularly, to waveguide multilayer holographic data storagesystems for providing a high throughput of data storage.

BACKGROUND

The logic of evolution of modern information technologies dictates anecessity to create data storage systems with a high informationcapacity, a high data rate and small access time, i.e. a high throughputsystem. Many researchers use the CRP (capacity-rate product) factor forthe throughput estimation where CRP=Capacity[GB]×Data Rate[Mbps] (HighThroughput Optical Data Storage Systems An OIDA Preliminary WorkshopReport April 1999. Prepared for Optoelectronic Industry DevelopmentAssociation by Tom D. Milster).

A more objective factor, being proposed for use in this invention, isCARP (capacity-access-rate product), which is the capacity in GB,divided by access time in ms and multiplied by the data rate in Mbps. Wehave CARP={C[GB]/A[ms]}×Data Rate[Mbps]. A comparison of CARP factorsgives the possibility to estimate objectively the advantages of any datastorage system in terms of throughput.

It is clear that a need exists for systems in future applications whereCRP>10⁵ and CARP>10⁶. That is, for example, a memory system with >1 GBinformation capacity, >100 Mbps data rate and <1 ms access time. At thesame time, it is clear that it is necessary to ensure a minimum qualityof recorded and readout signals, that is to provide a desired value ofthe signal/noise ratio and thereby to maintain a desired value of theerror probability.

Holographic methods are considered the most prospective for highthroughput data storage. More specifically, the data page orientedrandom access holographic memory is in the first place as a highthroughput system. However, there have been difficulties and problems inthe development of the high throughput system up to the present day. Thehigh data rate for optical data storage systems depends on the lightsource power, sensitivity of photodetector, the number of informationparallel input-output channels, and also on the conveying speed of thecarrier or optical reading head, when using a design with movingmechanical parts.

For holographic storage a large number of parallel data channels isprovided due to data presentation as two-dimensional pages of digitalbinary or amplitude data. Moreover, the highest data rate is providedwhen there are no moving mechanical parts, such as a rotating diskcarrier.

Short random access time of a memory system is a result of applying ahigh-speed addressing system such as electro- or acousto-opticaldeflectors and using a recording-reading schema, which provides fortransferring read images from different microholograms to aphotodetector without any mechanical movement.

Use of a volume information carrier in optical (including holographic)data storage for providing a high information capacity and highinformation density is well known, as in U.S. Pat. No. 6,181,665 issuedJan. 30, 2001 to Roh. But existing methods of optical (holographic) datastorage based on a volume carrier do not obtain high capacity and shortrandom access time simultaneously in accordance with the circumstancesindicated below.

There are several methods of volumetric holographic carrierapplications. The first is using angle multiplexed volume holograms,which provide for the superimposing of data pages of Fourier or Fresnelholograms in the volume photorecording medium. Each of the holograms isrecorded with a separate angle of the reference beam. The same angle ofthe readout beam is required for data page reading. Examples includeRoh, U.S. Pat. No. 6,072,608 issued Jun. 6, 2000 to Psaltis et al., U.S.Pat. No. 5,896,359 issued Apr. 20, 1999 to Stoll, and U.S. Pat. No.5,696,613 issued Dec. 9, 1997 to Redfield et al.

A second method is using encrypted holograms for holographic datastorage as in U.S. Pat. No. 5,940,514 issued Aug. 17, 1999 to Heanue etal. In the Heanue system orthogonal phase-code multiplexing is used inthe volume medium and the data is encrypted by modulating the referencebeam.

This method has a number of limitations. The main problem is adeficiency of the volumetric medium in meeting the necessaryrequirements. For example, ferroelectric crystals do not exhibitsufficiently great stability, and photopolymers have too large ashrinkage factor.

A third method is using holograms recorded in a multilayer medium asdescribed by “Holographic multiplexing in a multilayer recordingmedium”, Arkady S. Bablumian, Thomas F. Krile, David J. Mehrl, and JohnF. Walkup, Proc. SPIE, Vol. 3468, pp. 215-224 (1998) and by Milster. Oneor more holograms (a hologram matrix) are recorded in each layer of thevolume carrier. A readout of each hologram is made by a separate readingbeam. A limitation of this method is a low layer count, the number oflayers being limited by the noise from neighboring holograms located onother layers.

The last method is using waveguide multilayer holograms. See “Medium,method, and device for hologram recording, and hologram recording andreproducing device”, Mizuno Shinichi (Sony Corp.) JP09101735A2,Publication date: Apr. 15, 1997. Waveguide holograms are recorded inthin films of a multilayer carrier. Known methods of multilayeredwaveguide hologram recording and reading do not provide a high datadensity and small access time simultaneously.

International Publication No. WO 01/57602 discloses the recording ofholograms in a wave guide layer formed in a structure containingmultiple wave guide layers. An optical system allows the writing ofholograms in the wave guide layer and subsequent reading of the writtenholograms. However, the memory system does not provide a combination ofvery low access time and high data density simultaneously because thedata carrier tape or data storage card moves during readout. Anymechanical movement in a data storage system results in a relativelylong data access time.

The analysis of known methods and apparatus in the field of holographicdata storage permit to draw a conclusion: at the present time there isno high throughput holographic data storage system approach providing ahigh value of the CARP factor.

It is an objective of this invention to provide a holographic storagesystem with a high CARP factor.

SUMMARY

The present method offers an integrated approach to solving a problem ofproviding a high information capacity, high data rate and short accesstime simultaneously. The required characteristics of a system areprovided by a tightly bounded information carrier construction techniqueand new methods of data accessing, reading and recording.

The present invention includes a multilayer waveguide holographiccarrier, a multilayer waveguide holographic data storage system, amultilayer waveguide hologram reading method with random data access,and a process and apparatus for recording matrix waveguide hologramlayers and assembling a multilayer carrier. The multilayer wave guidehologram reading method incorporates an electronic moving windowprovided by a spatial light modulator (SLM) or charge coupled device(CCD) on the surface of the multilayer wave guide. The hologram pitch isrelated to the SLM or CCD element size.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention itself both as to organization and method of operation, aswell as objects and advantages thereof, will become readily apparentfrom the following detailed description when read in connection with theaccompanying drawings:

FIG. 1 a shows a multilayer waveguide holographic carrier with endsurface couplers for a reference beam;

FIG. 1 b shows a multilayer waveguide holographic carrier withdiffraction grating couplers for a reference beam;

FIG. 2 a illustrates a method of putting a reference beam into awaveguide layer of a data storage carrier through an end surface couplerand radiation from reconstructed holograms;

FIG. 2 b illustrates a method of putting a reference beam into awaveguide layer of data storage carrier through a diffraction gratingcoupler and radiation from reconstructed holograms;

FIG. 3 shows a data page image pattern to be stored holographically in afocusing plane;

FIG. 4 shows a hologram layer with a superimposed hologram;

FIG. 5 illustrates a system with random data access for retrievingholographically stored data from a multilayer waveguide carrier;

FIG. 6 illustrates a geometrical relationship between waveguideholograms in a hologram layer and a photodetector array;

FIG. 7 illustrates a system for retrieving holographically stored datafrom a multilayer waveguide carrier utilizing a phase conjugatereference beam;

FIG. 8 illustrates a system for superimposed waveguide hologram reading;

FIG. 9 illustrates a system for encrypted waveguide hologram reading;

FIG. 10 illustrates a system for waveguide hologram reading by a lasermatrix;

FIG. 11 represents a schematic view of a process and apparatus forrecording a matrix of waveguide Fourier (quasi Fourier) holograms in aphotorecording layer by using a diffraction grating coupler;

FIG. 12 represents a schematic view of a process and apparatus forrecording a matrix of waveguide Fourier (quasi Fourier) holograms in aphotorecording layer by using SLM disposed in a convergent beam;

FIG. 13 represents a schematic view of a process and apparatus forrecording a matrix of waveguide Fourier (quasi Fourier) holograms in aphotorecording layer by using a random phase mask;

FIG. 14 represents a schematic view of a process and apparatus forrecording a matrix of waveguide Fourier (quasi Fourier) holograms in alayer by using a small angle input of a reference beam;

FIG. 15 represents a schematic view of the single layer matrix waveguideFresnel hologram recording process and apparatus; and

FIG. 16 illustrates a system for multiplexed waveguide hologramrecording.

DETAILED DESCRIPTION

Multilayer Holographic Data Storage Carrier

FIGS. 1 a and 1 b show a multilayer holographic waveguide data storagecarrier 10. It comprises layer groups each containing a hologram layer11 i where i is the current layer index and cladding layer 12 i.Holograms 14 ik are located along row axis 01 ij where j is the currentrow index and k is the current hologram index. Holograms arenon-overlapping in each of the rows.

In the first variant shown in FIG. 1 a, hologram layer 11 i in eachgroup is at the same time a waveguide layer having end surface coupler15 i. In the second variant shown in FIG. 1 a, the hologram layer 11 iand waveguide layer 13 i with a diffraction grating coupler 16 i (seenin FIG. 1 b) in each of the groups are made separately and attached toeach other with an optical contact therebetween to provide transmissionof the guided wave into the hologram layer. In both variants there is acladding layer on the outer surface of the waveguide layer, with asimilar function to prior art cladding layers.

In FIGS. 1 a and 1 b h₌ is the size of a hologram in the row directionand d₌ is the pitch of a hologram in the row direction. h_(⊥) and d_(⊥)are the size and pitch of the holograms respectively in the transversedirection. h is the thickness of a hologram layer and d is the pitch ofthe layers.

As shown in FIGS. 2 a and 2 b, a readout beam 20 penetrates into awaveguide layer through coupler 15 i (or 16 i). Then, the readout beampropagates along respective row ij as a guided wave 21 ij andreconstructs radiation beams 22 ijk from all its hologramssimultaneously. Reconstructed radiation from each hologram propagatestowards an output surface 02 and is restricted in its spatial angle γ.

When holograms have a specified spatial angle γ of radiation, thehologram pitch p₌ between adjacent holograms is established so as toprovide an intersection of said radiation at plane 03 and in the areaabove this plane. All reconstructed radiation beams form focused datapage images at parallel plane 04.

FIG. 3 shows a data page image pattern 51 in the focusing plane 04. Datapixels 17 mn have sizes s₌, s_(⊥) and pitches t₌, t_(⊥) and are disposedas a 2-D matrix. m and n are current pixel indices along rows andcolumns respectively. All data page images have the same orientation. Mand N are quantities of data pixels in the respective direction.

FIG. 4 shows a hologram layer with superimposed holograms. The anglebetween non-parallel row axes 01 ij and 01′ij is α. Some hologramsrelating to different non-parallel intersecting rows are recorded so tobe at least partially superimposed. The angle between any of two nearestnon-parallel hologram rows is established to be not less than the angleselectivity of said superimposed holograms.

Readout Method and System

FIG. 5 illustrates a system for retrieving holographically stored datafrom the multilayer waveguide carrier. The system includes a multilayerholographic waveguide data storage carrier 10 and a layer and row accessunit 30. The layer and row access unit 30 is made up of a laser 31 forgenerating a beam of coherent radiation and a beam former 32 for forminga beam 24, which is deflected by angular deflector 33 and becomes beam25 passing through an optical element (lens) 34 to a selected layer 11 iand, through the respective coupler 15 i (or 16 i), into the selectedlayer along the required hologram row.

A hologram access unit 40 made in the form of a “moving window” isarranged in the region between planes 02 and 03 (see FIG. 2 a) andintended for separating radiation 22 ijk from any hologram 14 ijk togain access thereto and block radiation from other reconstructedholograms.

A multielement photodetector 50 faces towards the output surface 02 ofthe carrier, intended for receiving reconstructed radiation 22 ijk fromsaid hologram, disposed at plane 04 of focus of this radiation andoptically coupled with a pixel pattern 51 (see FIG. 3) of data stored bythe hologram.

Lastly, a computer 60 is connected through respective interface units tocontrol inputs of the layer and row access unit 61, hologram access unit62 and the photodetector 63 to control their coordinated operation.

FIG. 6 illustrates a geometrical relationship between waveguide hologram14 ijk in a hologram layer and photodetector array 50.

The photodetector array pixel quantity Q₌ in one direction, which isparallel to the hologram rows and data rows, must beQ₌=P₌/p₌≧(q₌−1)h₌/p₌+M=[h₌(q₌−1)+Mp₌]/p₌ where:

-   -   P₌ is the linear size of detector array along rows,        P₌=(q₌−1)h₌+Mp₌;    -   h₌ is the hologram pitch along a row;    -   q₌ is the number of holograms in the row;    -   p₌ is the pitch of detector pixels along a row; and    -   M is the number of pixels of readout data in a data page row.

Respectively, the photodetector array pixel quantity in other direction,which is perpendicular to hologram and data page rows, must beQ_(⊥)=Q_(⊥)/p_(⊥)≧h_(⊥)(q_(⊥)−1)/p_(⊥)+N, where:

-   -   Q_(⊥) is the linear size of detector array along columns;    -   h_(⊥) is the hologram pitch along a column;    -   q_(⊥) is the number of holograms in the column;    -   p_(⊥) is the pitch of detector pixels along the column; and    -   N is the number of pixels of readout data in a data page column.

L₌=(q₌−1)h₌+d₌ is the linear size of the hologram row in the selecteddirection. The pitch of data page image pixels is equal to or largerthan the detector pixel pitch in which case it is a whole numbermultiple of it.

FIG. 7 illustrates a system for retrieving holographically stored datafrom a multilayer waveguide carrier utilizing a phase conjugatereference beam 20*. In comparison with FIG. 5, a conjugate coupler 15*iis used and the photodetector is disposed at conjugate plane 04*.

FIG. 8 illustrates a system for superimposed waveguide hologram reading.Holograms from non-parallel rows are read by readout beams 20 and 20′having an angle • between them. An additional deflector is used in thelayer and row access unit to provide the required additional angulardeviation of reading beam 20 in a plane which is parallel to layer 11 i.For example, it is possible to use a rotated optical plate 35 inaddition to deflector 33 (made as a rotated mirror provided with arotary actuator controlled by computer through the respectiveinterface).

FIG. 9 illustrates a system for encrypted waveguide hologram reading. Amultichannel phase spatial light modulator 41 and cylindrical lens 36are used respectively for readout beam encoding (encryption) anddirecting the encoded beam 27 ij into waveguide layer 11 i.

FIG. 10 illustrates a system for waveguide hologram reading by a lasermatrix. Laser matrix 37 and optical fibers 38 ij are used for forming aseparate readout beam for each hologram row. The computer controls eachlaser of matrix 37 through an interface 65.

Waveguide Hologram Recording Process and Apparatus

Holograms can be recorded as Fourier (or quasi Fourier) or Fresnelholograms of a two dimensional matrix of digital (binary or multilevel)or analog signals. Hologram matrices are recorded on separate layers.Then the hologram layers (and waveguide layers when used separately) andcladding layers are sandwiched together forming an optical contactbetween them, thus producing the multilayer waveguide holographic datastorage carrier.

Fourier (or Quasi Fourier) Hologram Recording

FIG. 11 represents a schematic view of a process and apparatus forrecording a matrix of waveguide Fourier (or quasi Fourier) holograms ina photorecording layer by using a diffraction grating coupler. Amonochromatic light source, such as a laser, generates a beam ofcoherent radiation that is split into a first (signal) beam 70 and asecond beam which is used to form a reference beam 28 by optical means32, as shown in FIG. 11. A signal collimated beam 71 expanded bystandard optical means 80, such as lenses, passes through (or reflectsfrom) a spatial light modulator (SLM) 42. The data page is displayed bySLM 42. Computer 60 forms control signals which arrive at SLM 42 throughinterface 66. Beam 72, modulated in amplitude (or phase, orpolarization) according to the control signals, is focused at the plane06 near the photorecording medium 17 by an optical element (lens) 81following which it illuminates a local area of the photoredording medium17. Thus, this local area is illuminated by an image of the Fourier (orquasi Fourier) transformation function of the data page. The layer ofphotorecording medium 17 is laminated on an optically transparent hardsubstrate 18 (for example, glass).

Simultaneously, reference beam 28 is transformed by diffraction gratingreference beam coupler 73 into guided reference wave 29. Wave 29 thenilluminates the same local area.

A diaphragm 83 may be located close to the photorecording medium surfacefor preventing parasitic illumination of the photorecording medium.

The optical system for forming the transformed data page image to berecorded in the medium 17 may be realized by different methods, whichdepend upon the character of the readout beam as described below:

1) Readout beam is the analog of a reference beam.

In this case, the distance between plane 07 (where the optical element81 is located) and plane 08 (where the SLM 42 is located) is such thatthe reconstructed data page image will be located at the same distancefrom the photorecording medium as the distance from the hologram to thedetector plane of the readout device. At the same time, the pitch ofdata page pixel images must be equal to, or a whole number multiple ofthe pitch of photodetector pixels. This means, for example, that if thepitch of readout data pixel images at the plane 04 of photodetector 50(FIG. 6) is equal to the pitch of pixels displayed by the SLM, then adistance V between plane 08 and plane 07 is equal to the double focuslength (2F) of lens 81. F is the distance between planes 06 and plane07.

Different layers 11 i (FIG. 5) of multilayer holographic carrier 10 arelocated at different distances Gi (FIG. 6) from the photodetector plane04 (FIG. 5). Therefore, it is necessary to provide a condition:•Fi+Gi=constant. In this case, reconstructed data images from all layersof the carrier will have an identical scale.

Parallel plate 82 (FIG. 11) of optically transparent material (or aspecial phase compensator) is used to compensate for any difference inthe optical distance from different layers to the detector plane. Thethickness and refractive index of this plate must be such as to providean optical analog of carrier layers located between given layer 11 i,(FIG. 6) and photodetector plane 04 (FIG. 6).

2) Readout beam (such as 20*, FIG. 7) is phase conjugate to thereference beam.

In this case, as shown in FIG. 12, SLM 42 is in the convergent beam fromlens 81 in the immediate proximity of plane 07.

Note: the readout of these type of holograms does not provide for usingany image forming optics between hologram plane 01 i (FIG. 6) andphotodetector plane 04 (FIG. 6).

FIG. 13 represents a schematic view, which is the same as in FIG. 11,except for the use of a random phase mask 43 to provide a more uniformFourier image distribution in hologram recording plane 05 i. It ispossible to use a phase spatial light modulator as a phase mask 43.

Hologram Recording Procedure

As shown in FIG. 11, guided reference wave 29 propagates inphotorecording film layer 17 as in a waveguide. Simultaneously, themodulated signal beam (Fourier or quasi Fourier image) is directed alongthe line normal to the photorecording film layer. Holograms are recordedby sequentially shifting the photorecording layer after each recordingalong a distance in the specified direction which is equal to the pitchsize h₌ of the holograms to be recorded. Two-coordinate positioner 90 isused to make the shifting and is controlled by computer 60 throughinterface 67. The pitch (h₌ and h_(⊥), FIG. 1 a,b) of holograms must bedivisible by a whole number of photodetector pixels p₌ and p_(⊥) (FIG.6). Recorded holograms are arranged in hologram rows forming a matrix inthe photorecording layer.

FIG. 13 illustrates variants of the recording procedure using a carrier,which contains two different layers: a photorecording (photosensitive)layer 17 and a waveguide layer 19. In particular, the reference beam isdirected into waveguide layer 19 by a prism coupler 86.

As shown in FIG. 12 and FIG. 14, the reference beam 28 is directed at asmall angle β to the photorecording layer 17. If the photorecordinglayer does not have a hard substrate, it is possible to place this layerbetween optical plates 84 and 85 by using immersion layers 87 and 88having a refractive index close to that of the photorecording layer.

Fresnel Holograms Recording

In this case, the readout is to be made by the conjugate reference beam.The recording procedure is the same as described above, but, as shown inFIG. 15, optical elements, such as focusing lens 81 and collimating lens89, form a Fresnel image of SLM data page 42 in the hologram recordingplane 05 i.

Formation of a diffraction grating to couple the reference beam to thewaveguide layer.

Grating coupler 16 i (FIG. 1 b) is recorded by a holographic method onthe periphery of the photorecording layer 11 i (FIGS. 1 a, 1 b), whichis also a waveguide layer, or it is formed on the periphery of separatewaveguide layer 13 i (FIGS. 1 a, 1 b) by stamping, etching or otherknown methods.

Superimposed Hologram Recording

The recording procedure is the same as described above, but as shown inFIG. 16, at least two superimposed hologram 91 and 91′ are recordedsequentially in the overlapping area with different propagationdirections 29 and 29′ of the reference beam in the hologram recordingplane 05 i. A minimum angle • between reference beam directions isnecessary to provide the independent readout of holograms by theappropriate readout beam.

Encrypted Hologram Recording

The recording procedure is the same as described above, but thereference beam is formed by the same method as that used for forming areadout encoded beam 27 ij (FIG. 9).

Accordingly, while this invention has been described with reference toillustrative embodiments, this description is not intended to beconstrued in a limiting sense. Various modifications of the illustrativeembodiments, as well as other embodiments of the invention, will beapparent to persons skilled in the art upon reference to thisdescription. It is therefore contemplated that the appended claims willcover any such modifications or embodiments as fall within the scope ofthe invention.

1. A multilayer holographic data storage carrier, comprising at leasttwo groups of layers, each group containing: i) a layer (11 i) havingholograms (14 ijk) for keeping data to be stored, said hologramsarranged in one or more hologram rows, each of said hologram rows havingnon-overlapping holograms able to be reconstructed simultaneously by oneguided wave, ii) a waveguide layer (13 i) provided with a coupler (15i), and iii) a cladding layer (12 i) located on the outer surface ofsaid waveguide layer between adjoining layer groups, wherein each saidhologram (14 ijk) arranged in said one or more hologram rows is capableof reconstructing focused radiation directed therefrom towards a planaroutput surface (02) of said data storage carrier and restricted in itsspatial angle in order to provide for the spatial separation of itsradiation from that of other reconstructed holograms in a region abovesaid output surface (02) and thereby allowing access to data stored bysaid hologram (14 ijk), and wherein said holograms (14 ijk) in eachhologram layer are recorded to provide focusing of their respectiveradiation at a common focusing plane (04), which is parallel to theplanar output surface (02) of the data storage carrier and, which isdisposed in the area of intersection of radiation from said holograms.2. The data storage carrier according to claim 1, further including: a)a multi-element photodetector (50) facing towards an output surface (02)of said carrier, intended for receiving reconstructed radiation fromsaid hologram (14 ijk), disposed at or near a focusing plane (04) ofsaid radiation b) a hologram access unit (40) having a moving windowelectronically moveable without movement of said photodetector (50) orsaid hologram access unit (40), arranged in said region and intended forseparating radiation from any hologram to gain access thereto and blockradiation from other reconstructed holograms, c) a layer and row accessunit (15 i) for forming and directing a readout beam to a selected layerand, through a respective coupler, thereinto along at least one requiredrow, and d) a computer (60) having respective interface units (67)connected accordingly with control inputs of said layer and row accessunit (15 i) and said hologram access unit as well as with control inputsand outputs of said photodetector (50) for controlling their coordinatedoperation and for processing readout data
 3. The data storage systemaccording to claim 2, wherein when having specified spatial angles ofradiation from said holograms, a hologram pitch between adjacentholograms is established so to provide an intersection of said radiationin an area above said region and thereby permit the spatial separationof radiation reconstructed by the selected hologram from thatreconstructed by adjacent holograms arranged in each of said hologramrows.
 4. The data storage system according to claim 3, wherein saidholograms are arranged with an equal hologram pitch between each saidhologram in each said one or more hologram rows, while a similar spatialangle of said radiation is established for all of said holograms.
 5. Thedata storage system according to claim 2, wherein when having specifieda hologram pitch between adjacent holograms (d_(•), d₌), spatial anglesof radiation from said hologram are established so as to provide anintersection of said radiation in an area above said region and therebypermit the spatial separation of radiation reconstructed by the selectedhologram from that reconstructed by adjacent holograms arranged in eachof said hologram rows.
 6. The data storage system according to claim 5,wherein said holograms are arranged with an equal hologram pitch betweeneach said hologram in each said one or more hologram rows, while asimilar spatial angle of said radiation is established for all of saidholograms.
 7. The data storage system according to claim 2, wherein whenhaving at least two parallel hologram rows to be reconstructed togetherin any hologram layer, the row pitch between adjacent rows isestablished so as to provide an intersection of radiation from saidreconstructed holograms in an area above said region.
 8. The datastorage system according to claim 2, wherein when having at least twoparallel hologram rows each to be reconstructed separately in a hologramlayer, a row pitch between adjacent rows is established to be not lessthan the hologram size in a transverse direction to said hologram row.9. The data storage system according to claim 2, wherein when having atleast two non-parallel hologram rows each to be reconstructed separatelyin a hologram layer, at least two holograms relating to differentnon-parallel rows are recorded so as to be at least partiallysuperimposed.
 10. The data storage system according to claim 9, whereinan angle between any of two neighbouring non-parallel hologram rows isestablished to be not less than an angle selectivity of saidsuperimposed holograms.
 11. The data storage system according to claim2, wherein said holograms in each said hologram layer are recorded toprovide focusing their respective radiation at a specified distance intoone of different planes parallel to the flat output surface of thecarrier and disposed in an area of intersection of radiation from saidholograms.
 12. The data storage system according to claim 2, whereinsaid holograms in each said hologram layer are recorded to providefocusing their respective radiation at one of the different specifieddistances such that all radiation is focused in one and the same planeparallel to the flat output surface of the carrier and disposed in anarea of intersection of radiation from said holograms.
 13. The datastorage system according to claim 2, wherein each said hologram in eachsaid hologram layer is recorded to store a two-dimensional pixel patternof a data page.
 14. The data storage system according to claim 2;wherein said hologram layer in each of said groups is at the same timesaid waveguide layer.
 15. The data storage system according to claim 2,wherein said hologram layer (11 i) and said waveguide layer (13 i) ineach of said groups are made separately and connected to each other byan optical contact to provide transmission of said guided wave into saidhologram layer.
 16. The data storage system according to claim 2,wherein a row of data pixel images from each said reconstructed hologramat a receiving surface of the photodetector is aligned along therespective row of pixels of the latter and a pitch of said data pixelimages in this direction is established to be equal to, or a wholenumber multiple of, the photodetector pixel pitch in the same direction.17. The data storage system according to claim 16, wherein when saidpitch of data pixel images is equal to said photodetector pixel pitch,the center of each pixel image is disposed at about the center of thecorresponding photodetector pixel.
 18. The data storage system accordingto claim 17, wherein the photodetector (50) is disposed in an area ofthe intersection of radiation from said holograms and the number ofphotodetector pixels in said direction is established to cover datapixel images from all said holograms without moving the photodetector inthe focusing plane (04) and determined by an expression: Q≧[h(q−1)/p+M],where h—is the hologram pitch, q—is the number of holograms in thehologram row, p—is the photodetector pixel pitch, M—is the number ofdata pixel images in said direction.
 19. The data storage systemaccording to claim 2, wherein said hologram access unit (40) is made asa spatial light modulator having the control input and being intendedfor modulating intensity (or amplitude) of reconstructed radiationtransmitted therethrough.
 20. The data storage system according to claim18, wherein said spatial light modulator is disposed at the outputsurface (02) of the carrier.
 21. The data storage system according toclaim 2, wherein said window has the form of a slit aligned transverselyto a photodetector pixel row, covering all said hologram rows and havinga controllable width depending on the distance of the respectivehologram layer from the output surface of the carrier and the specifiedspatial angle of radiation from the respective reconstructed hologram.22. The data storage system according to claim 2, wherein said windowhas a rectangular form aligned by one of its sides along a photodetectorpixel row and having a controllable size both in the direction of saidphotodetector pixel row and in the transverse direction, saidcontrollable size being dependent on the distance of the respectivehologram layer from said output surface of the carrier and the specifiedspatial angle of radiation from the respective reconstructed hologram.23. The data storage system according to claim 2, wherein said layer androw access unit (30) comprises: a) a unit (31) for generating andforming a beam of coherent radiation, said unit having a control inputdesignated as the first control input of the layer and row access unitand connected via said interface unit (67) with the computer (60); b)angular deflecting means (33) for deflecting the beam of coherentradiation in a plane perpendicular to said hologram layers and in theplane transverse thereto to gain access to a selected layer and arequired hologram row respectively, said angular deflecting means havinga control input designated as the second control input of the layer androw access unit and connected via said interface unit (67) with thecomputer; and c) an optical element having an input coupled to saidangular deflecting means and an output conjugated optically with acoupler of the selected layer and intended for converting angularvariations of a deflected beam into parallel shifting of a readout beamat its output and directing the readout beam through said coupler intothe selected layer along the required hologram row.
 24. The data storagesystem according to claim 2, wherein said layer and row access unit ismade as a set of lasers each having output optics and a control inputbeing the respective control input of the layer and row access unit (30)and connected via the respective interface unit with the computer (60),said optics being conjugated by optical means with a coupler (15 i) ofthe respective hologram layer for directing the readout beam thusproduced through said coupler into said respective hologram layer alongthe corresponding hologram row.
 25. The data storage system according toclaim 24, wherein said optical means is an optical fiber.
 26. A methodof reading out data stored in a multilayer holographic data storagecarrier (10), comprising: a) forming a readout beam to be used forgaining access to data stored in the carrier having at least two groupsof layers, each group containing: i) a layer having holograms (11 i) forkeeping data to be stored, said holograms being arranged in one or moreof hologram rows, each of said hologram rows having non-overlappingholograms able to be reconstructed simultaneously by one guided wave,ii) a waveguide layer (13 i), and iii) a cladding layer (12 i) locatedon the outer surface of said waveguide layer between adjoining layergroups, b) directing the readout beam into the waveguide layer (13 i) ofthe selected group along at least one hologram row for gaining access tothe selected hologram layer (11 i) and the required hologram row andreconstructing respective holograms, c) selecting radiation from one ofsaid reconstructed holograms for allowing access to data stored therein,said selecting step being done by a moving window in a non-movingspatial light modulator and d) receiving reconstructed radiation from aselected hologram for processing read out data, wherein said step ofselecting radiation is carried out by spatial separating of radiationreconstructed by the selected hologram from that reconstructed byadjacent holograms arranged in each of said hologram rows, saidseparating is carried out in a region above an output surface of saidcarrier due to that each said hologram arranged in each of said one ormore hologram rows is capable of reconstructing focused radiationdirected therefrom towards said output surface of said carrier andrestricted in its spatial angle to provide an intersection of saidradiation from reconstructed holograms in an area above said region,while said step of receiving reconstructed radiation is carried out insaid area at or near a focusing plane (04) of said radiation.
 27. Amethod of reading out data according to claim 26, wherein said hologramlayer in each of said groups is at the same time said waveguide layer.28. A method of reading out data according to claim 26, wherein saidstep of receiving reconstructed radiation is carried out by means of amultielement photodetector (50) disposed in said area at the focusingplane of said radiation and oriented so that a row of data pixel imagesfrom each said reconstructed hologram at a receiving surface of thephotodetector is aligned along a pixel row of the latter, while thenumber of photodetector pixels in this direction is established so as tocover data pixel images from all said holograms without moving thephotodetector in the focusing plane and determined by an expression:Q≧h(q−1)p+M, where h—is the hologram pitch, q—is a quantity of hologramsin the hologram row, p—is the photodetector pixel pitch, M—is a quantityof data pixel images in said direction.
 29. A method of reading out dataaccording to claim 26, wherein the step of spatial separating ofradiation reconstructed by the selected hologram is carried out by usinga moving window arranged in said region and capable of changing itsposition and size for transmitting this radiation therethrough andblocking radiation from other reconstructed holograms.
 30. A method ofreading out data according to claim 29, wherein said window is carriedout by means of a spatial light modulator intended for modulatingintensity (or amplitude) of reconstructed radiation transmittedtherethrough.
 31. A method of reading out data according to claim 29,wherein when using a multielement photodetector for receiving saidreconstructed radiation, said window has the shape of a slit alignedtransversely to a photodetector pixel row, covering all said hologramrows and having a controllable width depending on the distance of therespective hologram layer from the output surface of the carrier and thespecified spatial angle of radiation from the respective reconstructedhologram.
 32. A method of reading out data according to claim 29,wherein when using a multielement photodetector for receiving saidreconstructed radiation, said window has a rectangular shape aligned byone of its side along a photodetector pixel row and having acontrollable size both in the direction of said pixel row and in thetransverse direction, said controllable size being dependent on thedistance of the respective hologram layer from said output surface ofthe carrier and the specified spatial angle of radiation from therespective reconstructed hologram.
 33. A method of reading out dataaccording to claim 26, wherein when any of said holograms are encrypted,a read out beam is composed of a number of rays having differentdirections corresponding to those of reference rays used for recordingthe respective encrypted hologram.